1. Field of the Invention
The present invention relates to a skew correction apparatus or, in particular, to a skew correction apparatus for reducing the skew amount constituting a phase shift at the receiving end of a data transmission system.
2. Description of the Related Art
FIG. 1 is a block diagram showing the most basic parallel data transmission system according to the prior art. In FIG. 1, data DATA0 to DATA3 and a byte clock signal CLK are transmitted in parallel through a plurality of transmission channels 12-1 to 12-5 between a transmitter 10 and a receiver 11. The DATA0 to DATA3 are each an 8-bit serial data. In the case where variations occur in the device characteristics or the cable transmission delay time between the transmission channels 12-1 to 12-5, the skew constituting a phase shift occurs between transmission channels. As a result, the problem is posed that the increase in data transmission rate and the number of bytes are restrained.
FIG. 2 is a block diagram of a conventional serial bundle parallel data transmission system intended to solve this problem. This system is known, for example, as the Infiniband transmission system proposed by such companies as Sun, Intel and IBM. In FIG. 2, a transmitter 21 includes a clock gate 211 for distributing a byte clock signal (B CL) to encoders and parallel-serial converters, four encoders 212 for converting 8-bit data into 10-bit data for improving the correction quality by avoiding continuous 0s in the data, four parallel/serial converters 213, and four electro-optic converters 214. The receiver 22, on the other hand, includes four opto-electric converters 221, four clock recovery circuits 222, four serial/parallel converters 223 and four decoders 224 corresponding to the four channels. The outputs of the four decoders 224 are input to a deskew circuit 225.
The encoders 212 of the transmitter 21 are each supplied with a clock signal from the clock gate 211, and convert the parallel data DATA0 to DATA3 with one byte constituted of 8 bits into parallel data with one byte constituted of 10 bits. Each of the parallel/serial converters 213 converts parallel data into serial data byte by byte. The serial data are converted from an electrical signal into an optical signal by the electro-optic converters 214 for lengthening the transmission distance and transmitted through an optic fiber.
The optical signal received by the receiver 22 through the optic fiber is converted into an electrical signal by the opto-electric converter 221. The clock signal is reproduced by the clock recovery circuits 222 and converted into a parallel signal by the serial/parallel converter 223. The decoder 224 reproduces the parallel data with one byte constituted of 8 bits from the parallel data with one byte constituted of 10 bits. With the increase in transmission rate (the Infinitiband described above has a width of 400 ps per bit), skewless transmission has physically become impossible and a deskew circuit 225 is required of the data receiver 22. The skew contained in the 8-bit parallel data reproduced is removed by the deskew circuit 225.
Generally, the following described deskew systems are conceived.
(1) At the time of initial set-up of the apparatus (transmission system), a predetermined data pattern for skew correction is transmitted and a delay value of the delay circuit is tuned for each transmission channel at the receiving end.
(2) Before starting the data transmission, a predetermined data pattern for skew correction is transmitted and a delay value is set for each transmission channel in the deskew circuit at the receiving end. In a method of setting a delay value, several stages of shift register are provided for each transmission channel, and the receiving data are received only after passing through the shift registers so that the data patterns for skew correction of the transmission channels are in phase with each other.
In the prior art described above, the deskew work is required to be carried out again in the case the skew conditions undergo a change due to the variations of the device characteristics with temperature or the change in cable layout after initialization for deskew. From the viewpoint of the data transmission quality of the computer system, however, it is not desirable to carry out the deskew work after an error occurs. In order to avoid a data error due to a skew, therefore, the deskew work must be carried out at regular time intervals. The deskew work, however, suspends the data transmission and therefore reduces the data transmission capacity, resulting in a deteriorated system performance.